The bill is supported by state legislators from both parties, and has support from the Michigan State Police.

The idea is to give police an analogue to the breath tests used to check whether drivers are drunk.

Drivers wouldn't be arrested just for failing the proposed saliva tests -- instead, the tests would be used to add to the evidence used to justify an arrest. But researchers who have studied the test say the results are inconsistent, especially when applied to regular marijuana users.

Michigan has 100,000 residents allowed to use medical marijuana."I don't know what the level [for impaired driving] is going to be on the Michigan tests, but I suspect you'll effectively prohibit many people from driving," Brett Ginsburg, an associate professor at the University of Texas Health Science Center at San Antonio, told the Free Press.

The issue is that while alcohol "pretty much permeates the entire body, all at once," according to Ginsburg, THC, the psychoactive component in marijuana, affects the brain through the nervous system.

Saliva, obviously, falls outside the nervous system, making it a bad indicator of how behavior might be impaired. Advocates have raised the issue with the lawmakers pushing the bill.

In response, state Rep. Dan Lauwers (R), the bill's sponsor, has said he would support an amendment to let people with medical marijuana licenses decline the test.

According to the Free Press, police in Los Angeles are currently using the tests in field trials, to figure out if they could hold up in court.